1. Field of the Invention
The present invention relates to a method for installing an inner blade part of a wind turbine blade, said method comprising the steps of hoisting said inner blade part and arranging said inner blade part in relation to said hub flange.
2. Description of Related Art
The invention further relates to a lifting bracket suitable to be used during installation of an inner blade part of a wind turbine blade, e.g., a inner blade part for a partial pitch wind turbine blade comprising an inner blade part and an outer blade part, said inner blade part comprising a first flange for connection to a hub flange at a wind turbine hub and a second flange for connection to an outer blade part, said flanges all comprising a number of bolts and/or bolt holes.
Finally, the invention relates to the use of a lifting bracket and a method as mentioned above.
It is well known that blades for modern wind turbines have to be handled during production, transport and installation and as such blades easily can have a length of 40 to 60 meters or in the near future up to approximately 80 meters, handling can be quite a challenge.
Most of the wind turbines produced are so-called pitch controlled or active stall controlled. The blade is installed at some kind of bearing at the hub and via said bearing it is possible to rotate the blade about a central axis of the flange at the hub. When a wind turbine blade has to be installed at the hub, the blade will be hoisted and positioned at said hub and typically the bearing will be rotated and/or adjusted into the right position in relation to bolt holes in a flange at the root end of the blade. Typically, the right position is the nearest bolt hole as the blade can be positioned in its final position after installation by activating the pitch system. In this way many blades have been installed over the years and the method works very well.
Due to the fact that the blades are becoming longer and longer, to be able to produce more power, several attempts to produce dividable blades for a wind turbine have been made. Until now, nobody has come up with a solution that really works. As the blades during use are facing quite drastic deformations there is a large challenge in designing the interface on the respective parts of a dividable blade.
One way of dealing with the problems incurred by manufacturing and handling long blades is to design a wind turbine having blades with partial pitch. Such blades are typically stall controlled on the innermost part, also called the inner blade part, and pitch controlled at the outermost part, also called the outer blade part. There is thus provided a pitch bearing at the joint between the inner and the outer blade part. Such a blade will typically be divided between ⅓ and ½ of the length when measured from the root end of the blade and towards the tip end. The inner blade part is designed as a rigid component that is suitable for having a pitch bearing installed at the interface towards the outer blade part.
When installing the outer blade part to the inner blade part the same method as discussed above can be used with no problems. When it comes to installing the inner blade part to the hub, which typically will take place before installing the outer blade part, a number of other problems arise. As there is no pitch bearing between the hub and the inner blade part to rotate the blade into any position, the blade can not be installed in just any position, it has to be aligned according to and installed into the exact bolt holes. Such a handling will often take place at the erection site of the wind turbine. Even though the blade is divided into an inner blade part and an outer blade part, the two parts are still big, heavy and are not easy to handle. The diameter at the root end will typically be 2.5 to 3.5 meters or even more and the inner blade part can e.g., have a weight of approximately 10 to 20 metric ton or more and a length of e.g., 20 to 40 meters or even more. It is thus clear that this is a job for a crane or even for two cranes. But still the inner blade part has to be positioned at the exact bolt holes.